1. Field
The embodiments described below relate generally to systems for delivering radiation treatment. More specifically, some embodiments are directed to treatment verification systems used in conjunction with such delivery.
2. Description
Conventional radiotherapy systems direct a beam of photon, proton, neutron, or other radiation toward a target volume of a patient. The radiation destroys cells within the target volume by causing ionizations within the cells or other radiation-induced cell damage.
Isocentric radiotherapy is typically delivered by a therapeutic radiation source integrated into a rotatable gantry. The gantry rotates around a horizontal axis such that a radiation beam emitted from the therapeutic radiation source passes through a same volume of space (i.e., an isocenter) at each angle of rotation. A target volume of a patient is therefore positioned at the isocenter prior to emission of the beam and rotation of the gantry. Due to physical constraints, isocentric treatment is particularly suited to target volumes located above the chest region.
Multi-jointed robotic arms are typically used to deliver non-isocentric radiation treatment. Such arms include an integrated therapeutic radiation source and provide more flexible positioning with respect to a patient in order to deliver treatment radiation from any angle to a target volume located virtually anywhere within the patient. Non-isocentric radiation treatment may therefore irradiate the target volume from fewer external positions than those used during isocentric radiation treatment. Accordingly, non-isocentric treatment may be required in order to avoid irradiating sensitive healthy structures.
In both isocentric and non-isocentric treatment modes, the target volume and the radiation source must be registered to one another in three-dimensional space so as to ensure delivery of the radiation according to a treatment plan. Errors in radiation delivery can result in low irradiation of tumors and high irradiation of sensitive healthy tissue. Positioning accuracy may be particularly problematic in the case of non-isocentric treatment, since up to six degrees of freedom may be available. All six degrees of freedom may be necessary to irradiate a certain spatial position (x, y, z) within the target volume from an arbitrary direction orientation (yaw, pitch, roll).
The GammaKnife® radiosurgical system by Elekta employs Cobalt-60 radiation sources which are fixed in space. An anatomical target (e.g., a volume within a patient's head) is moved and registered to the focal point of radiation sources. This scenario requires the rigid fixation of the patient's skull within a pin based stereotactic frame, with substantial discomfort to the patient. By relocating the frame within the system, the patient's skull is repeatedly repositioned relative to the fixed radiation reference frame based on a pre-calculated radiation plan.
Some radiosurgery systems avoid such fixed-frame positioning by using external markers on the patient as well as markers internal the patient. Spatial correlation between the external markers and the internal markers is established by an X-ray image showing both sets of markers. Positions of the external markers are tracked during treatment and, based on the predetermined spatial correlation, corresponding positions of the internal markers are determined. The accuracy of the latter determination is limited due to the variability of the actual correlation between the external and internal markers.
What is needed is improved determination of a target position within a robotic stereotactic radiosurgery system.